1. Field of the Invention
The present invention relates to a rotating machinery controller for controlling a rotation-related physical quantity of a rotating machinery by operating switching devices of an inverter to supply electric power to the rotating machinery.
2. Description of the Related Art
Rotating machinery controllers of the type concerned are known. One example of such known controllers is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 10-174453. The disclosed rotating machinery controller is arranged to perform instantaneous current value control in which switching devices of an inverter are operated based on a magnitude relation of upper and lower limits of a hysteresis region determined by a command current to a three-phase rotating machinery and an actual current so that the actual current is controlled by a feedback control system with respect to a required current to generate a desired torque.
Another controller is also known, which in a high output torque range or a high rotational speed range of the rotating machinery, performs rectangular wave control to operate the switching devices of an inverter in such a manner that an On-state and an Off-state each occur one time per one period of electrical angle of the rotating machinery. According to the rectangular wave control, it is possible to suppress power loss in the high power and high speed ranges and also to improve the voltage utilization factor which is defined as the ratio of the root-mean-square phase-to-phase voltage to the input voltage of the inverter.
Still another rotating machinery controller is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2003-235270.
However, in the instantaneous current value control described above, as the speed of the rotating machinery goes up toward a high rotational speed range, the actual current tends to deviate from the command current, making it difficult to generate the desired torque. During normal operation, the input voltage is larger than the back electric motive force (emf) and, hence, the rate of change of the actual current is sufficiently larger than that of the command current. Accordingly, the actual current can follow or tack the command current while changing little by little. In the high rotational speed range, however, phase shifting between the actual current flowing through the three-shape rotating machinery and the command current becomes significant. This is because the difference between the input voltage of the inverter and the back emf of the three-phase rotating machinery becomes small in the high rotational speed range, the rate of change of the actual current flowing through the three-phase rotating machinery is at the same level as the rate of change of the command current.
When the command current and the actual current are out of phase, a torque actually output from the three-phase rotating machinery deviates from the desired torque.
On the other hand, the rectangular wave control is able to control the output torque of the rotating machinery toward the desired torque even in the high rotational speed, high output torque ranges. However, in the rectangular wave control, the current flowing through the rotating machinery is controlled by an open-loop control system rather than a feedback control system. Thus, the rectangular wave control has a potential risk of allowing generation of an excessively large current. When such excessively large current flows through the rotating machinery, the reliability in operation of the switching devices of the inverter is deteriorated.